Pigments generally exhibit visit color tone and high coloring power, and they are widely used in many fields. Examples of use applications in which pigments are used include paints, printing inks, electrophotographic toners, ink-jet inks, and color filters. Pigments are important compounds indispensable in everyday life at the present time. General properties, classification by use, and other aspects of pigments are described in, for example, “Dispersion Stabilization of Pigment and Surface Treatment Technique/Evaluation”, Technical Information Institute Co., Ltd., 2001, pp. 123-224. In connection with the above-mentioned use applications, examples of pigments that must provide high performance, and that are of particular importance in practical use, are ink-jet ink pigments, and color filter pigments.
As the coloring material for ink-jet ink, dyes have been used, but they have drawbacks as to water resistance and light resistance. To overcome the drawbacks, pigments have come to be used. Images obtained from pigment inks have remarkable advantages of superior light resistance and water resistance compared with images obtained from dye-based inks. However, the former images have the problems that the pigment is not easily formed uniformly or pulverized into fine-particles of a nanometer size (i.e., monodispersion), which can permeate pores in the surface of paper, and then the pigment in the image are poor in close bindness to the paper.
With an increase in the number of pixels in digital cameras, there is a need for a color filter used in a CCD sensor to be made thinner. In the color filter, an organic pigment is used. Since the thickness of the filter depends largely on the particle diameter of the organic pigment, there has been a need to produce monodisperse and stable fine-particles of a nanometer-size level.
In general, the methods to produce fine particles are roughly classified into the breakdown method, in which fine particles are produced from a bulk material by pulverization or the like, and the build-up method, in which fine particles are produced by particle-growth from a gas phase or liquid phase, as described, for example, in “Experimental Chemical Lecture, 4th Edition,” edited by the Chemical Society of Japan (Maruzen Co., Ltd.), vol. 12, pp. 411-488. The pulverizing method, which has been widely used hitherto, is a fine-particle-producing method having high practicability, but it has various problems, such as that its productivity is very low in producing organic material particles of nanometer size, and that the materials to which the method can be applied are limited. In recent years, investigations have been made to produce organic material fine-particles of nanometer size by the build-up method.
One of the methods recently disclosed is to use a supercritical fluid or subcritical fluid to make azo pigments, which are organic pigments, into fine particles (for example, JP-A-2002-138216 (“JP-A” means unexamined published Japanese patent application)). That is, this method is one dissolving the pigment into a supercritical fluid or subcritical fluid, and then returning the solution to a condition of ambient temperature and normal pressure, to grow crystals, thereby producing fine particles. Performance of this method causes such problems: First, it requires an apparatus that can realize very high temperature and pressure, close to supercritical temperature and pressure; and second, the organic compounds generally decompose easily under such conditions.
A second method of the above-mentioned methods uses a microjet reactor: a micro-chemical process technique that will be described later, to produce fine particles (for example, JP-A-2002-146222, 2002-155221, and 2002-161218). This is a method of introducing a solution, in which a pigment is dissolved, and a precipitation medium solution, into two nozzles that are opposite to each other and that have different micrometer sizes, at high pressure (for example, 5 MPa) by means of a pump; and introducing a gas (such as compressed air) perpendicularly into a portion where the jet flows of the two solutions collide with each other, so as to discharge a suspension of the pigment by the gas flow (flow rate, about 0.5 m3/h). The methods described in JP-A-2002-146222 and JP-A-2002-161218, which are examples of this method, correspond to a breakdown method in which pigment suspensions are caused to collide with each other in a chamber, thereby making the pigment particles finer. On the other hand, the method described in JP-A-2002-155221 is to spray a solution of a pigment and a precipitation medium into a chamber, to precipitate the pigment, thereby producing fine particles, which method can be regarded as a build-up method. This method adopts the following contrivance: Particles are generated in a very small space, of a micrometer scale, and the particles are immediately taken out from the apparatus, thereby preventing the apparatus from being blocked by the pigment fine-particles. Thus, this method is preferred to obtain fine particles having a narrow particle size distribution. However, the method has such problems as that the contact time for the two solutions is not easily controlled, and thus subtle reaction control is difficult.
A third method is to bring a solution, in which an organic pigment is dissolved, gradually into contact with an aqueous medium, to precipitate the pigment (a so-called co-precipitation method (re-precipitation method)), in which a dispersing agent is incorporated into either of the solutions, thereby producing stable, fine particles (JP-A-2003-26972). This method enables producing particles of a nanometer size easily. However, when the system is scaled up, the sizes of the particles are scattered, and needle particles are apt to generate. Therefore, even if the generated particles are observed as single-nanometer particles in a particle-measuring apparatus, observation of the particle shape thereof with a transmission electron microscope shows the particles to be considerably long needle particles. Thus, the particles are unsuitable for fine particles for ink-jet ink, which are preferably spherical.
A known method, positioned between the build-up method and the breakdown method, is a conditioning method, in which coarse particles are heat-treated to adjust the particle diameter thereof. In a method recently disclosed, an organic pigment is conditioned in a micro-reactor, using an idea of the micro-chemical process technique, which will be described later (JP-A-2002-30230). JP-A-2002-30230 describes that a suspension of a liquid pigment precursor (a suspension in which a pigment having a broad particle size distribution is suspended in a solution) is continuously introduced into a micro-reactor and heat-treated, thereby the pigment crystalline fine-particles in the suspension undergo phase-change; and at the same time to the phase-change, particles having an average particle size larger than that of the precursor, and having a narrow particle size distribution, can be produced. This method advantageously produces particles having a narrow particle size distribution, but it has the drawback that particles favorably having a small particle diameter, among the precursor particles, are also made larger.
In recent years, attention has been paid to a technique in which a reaction channel, having a microscopic channel sectional area, is used to conduct chemical reaction; that is, the so-called “micro-chemical process technique,” since chemical reaction can be effectively conducted. The “micro-chemical process technique” is substance-producing technique and chemical analysis technique using a chemical and/or physical phenomenon that is caused in a micro-channel, which is formed on a solid substrate by micromachining technique and the like, and which has a width of several micrometers to several hundred micrometers.
A general method of producing a disazo condensed-type pigment is described, for example, by W. Herbst and K. Hunger, in “Industrial Organic Pigments, Production, Properties, Applications, —Second Completely Revised Edition,” VCH A Wiley Company, 1997, pp. 595-630. On the other hand, as a method of producing a pigment in a micro-reactor based on a micro-chemical process technique, a method of producing a disazo condensed-type pigment or a diketopyrrolopyrrole-series pigment is disclosed in, for example, JP-A-2002-38043 and JP-A-2002-12788. These can be regarded as one type of a build-up method. In the description of JP-A-2002-38043, the step up to the synthesis of the disazo-type pigment is conducted in a micro-reactor. However, the starting compound itself has low solubility, and therefore the compound is introduced as a suspension, into the micro-reactor. This may be highly apt to cause blockage of the channel if conditions to be controlled are incorrect. Thus, it is unavoidable to say that there is difficulty from the viewpoints of reproducibility and continuous production.